The present invention relates generally to a heating, ventilating, and air conditioning system. More particularly, the present invention relates to a method and system for controlling a heating, ventilating, and air conditioning system.
Heating, ventilating, and air conditioning (HVAC) systems are used in both warm and cold climates to control the temperature of the air in a building or zone or zones within a building. An HVAC system typically includes a fan, a heating unit, and a cooling unit. The HVAC system may be operated in one of three modes: a heating mode to heat the enclosure, a cooling mode to cool the enclosure, or a ventilation mode to ventilate the enclosure.
Typically, a supply duct and a return duct connect the HVAC system to the zone or zones being conditioned. The fan operates to push air through the supply air duct and into the zone(s). Air is circulated through the zone(s) and back to the HVAC system through the return duct. The HVAC system may also include an outdoor air damper, commonly referred to as an economizer, that can be selectively opened to varying degrees, to allow outside air to mix with the return air to provide fresh air to the zone(s).
The operation of a HVAC system is governed by a control system, typically including a thermostat and associated programmable control hardware and software that sense parameters and control the various components of the particular HVAC system. The control system allows a user to select the operating mode of the HVAC system. In addition, the user may select a desired temperature for the zone or zones. The control activates the heating and cooling units in the HVAC system to maintain the desired temperature in the zone(s).
If the HVAC system is set to operate in the cooling mode and the temperature of the area exceeds the desired temperature, the HVAC system will activate the fan and the cooling stage. The fan blows air through the cooling unit and into the zone(s), thereby reducing the temperature of the air in the zone(s). Once the zone(s) are cooled to below the set temperature, the control device will turn off the cooling unit, or lower the stage of the cooling system, if it is a multi-stage system. Similarly, if the HVAC system is set to operate in the heating mode and the temperature of the zone(s) drops below the set temperature, the control device will activate the heating unit to warm the zone(s). The fan blows air through the heating unit and into the zone(s), thereby warming the zone(s). When the temperature rises above the desired temperatures the control device will shut off the heating unit, or lower the stage or capacity of the heating system, if it is a variable capacity or multi-stage system.
In certain HVAC systems, such as a constant volume HVAC system servicing a plurality of zones at a relatively constant volume of air flow, the unit will switch to the ventilation mode after the heating or cooling needs of the enclosure are satisfied. In the ventilation mode, the economizer is opened to allow outdoor air to enter the HVAC system. The fan mixes the outdoor air with the return air to ventilate the zone(s). The amount of air that is allowed through the economizer is governed by local building codes. According to these codes, the damper must typically be open at least 15% to 35%.
If there is a large temperature difference between the outdoor air and the air in the enclosure, the ventilation mode may cause discomfort within the zone or room. If, for example, the HVAC system is located in a hot climate where the outdoor temperature is much greater than the temperature of the zone(s), the mixture of the hot outdoor air with the air in the return duct results in supply air that is much warmer than the air in the zone. When this warm supply air enters the relatively cool enclosure, the flow of this warm air will be uncomfortable to persons near the supply vents and may cause hot spots in the room or zone. In addition, adding this warm supply air to the zone(s) will cause the temperature of the enclosure to rise rapidly. Similarly, if the outside air is very cold relative to the indoor temperature, the air supplied in the ventilation mode will be too cold and will cause similar problems in the opposite extreme.
In light of the foregoing there is a need for a method and system for controlling a heating, ventilating, and air conditioning system to control the temperature of the supply air in the ventilating mode.